This invention relates to thin foils or strips made of refined aluminum with a purity exceeding 99.9%, and which are subjected to an etching surface treatment designed to increase their specific area, and are then used in the manufacture of anodes for electrolytic capacitors, and particularly high voltage capacitors.
A lot of work has been carried out to study the effect of some trace elements in aluminum on the density of the pores obtained during the etching treatment and the capacitance of the capacitor made using foils of this aluminum. The role of lead, indium and boron in particular has been demonstrated.
The effect of lead is mentioned for the first time in U.S. Pat. No. 3,997,339 by Siemens published in 1976 that describes the influence of antimony, barium and zinc between 5 and 220 ppm, and the influence of lead and bismuth up to 0.5 ppm, and calcium and chromium up to 2 ppm. The patent application JP 58-42747 filed by Toyo Aluminium mentions the favorable effect of an indium content of 0.1 to 1 ppm for etching. The article by K. Arai, T. Suzuki and T. Atsumi xe2x80x9cEffect of Trace Elements on Etching of Aluminum Electrolytic Capacitor Foilxe2x80x9d Journal of the Electrochemical Society, July 1985, studies the influence of traces of bismuth and boron on the morphology of etching and the capacitance.
Some work has shown that favorable elements must be concentrated in a zone close to the surface, if they are to be fully efficient. Thus, patent application JP 57-194516-A by Toyo Aluminium published in 1982, demonstrates the beneficial effect on the etchability of a concentration of between 50 and 2000 ppm of lead, bismuth and/or indium content in the surface zone down to a depth of 0.1 xcexcm.
Patent EP 0490574 by Showa Aluminium published in 1992 describes the concentration at different contents of Fe, Cu, Zn, Mn, Ga, P, V, Ti, Cr, Ni, Ta, Zr, C, Be, Pb and In elements, either at the interface between the surface oxide layer of the foil and the body of the foil, or within the oxide layer. The concentration ratio of elements in the concentration zone and elements in the core of the foil, as measured with an ion probe, is between 1.2 and 30.
U.S. Pat. No. 5,128,836 by Sumitomo Light Metal published in 1992 describes the concentration of Pb, Bi and/or In at a content of between 10 and 1000 ppm in a sub-surface zone at a depth of between 0. 1 and 0. 2 xcexcm. Proposed methods to encourage surface migration of the various elements are either heat treatments, for example final annealing under special conditions, or physical deposits such as cathodic sputtering or ion implantation.
Finally, it is known that low capacitances are obtained when etching is not uniform at the surface of the foil. The link between these etching heterogeneities and the surface distribution of elements like Pb, Bi or In has not been clearly established, as can be seen in the articles by W. LIN et al. xe2x80x9cThe Effect of Lead Impurity on the DC-Etching Behaviour of Aluminum Foil for Electrolytic Capacitor Usagexe2x80x9d Corrosion Science, vol. 38, No. 6, 1996, pp. 889-907, and xe2x80x9cThe Effect of Indium Impurity on the DC-Etching Behaviour of Aluminum Foil for Electrolytic Capacitor Usagexe2x80x9d, Corrosion Science, vol. 39, No. 9, 1997, pp. 1531-1543.
The purpose of the invention is to improve the beneficial effect of the surface concentration of Pb, B and In elements on the etchability of thin foils of refined aluminum for electrolytic capacitors. It is based on the demonstration of the favorable effect of a uniform distribution of these three elements at the surface of the foil.
The purpose of the invention is a thin foil of refined aluminum with a purity exceeding 99.9% of aluminum designed for the manufacture of anodes of electrolytic capacitors comprising at least one of the elements Pb, B and In with an average total content (by weight) of between 0.1 and 10 ppm (and preferably between 0.5 and 5 ppm) for which the distribution of these three elements in the surface zone at a depth of 0.1 xcexcm is such that their signal current obtained by ionic analysis has a dispersion ratio Rd=(Imaxxe2x88x92Imin)/Iaverage less than 5, and preferably less than 2.